1. Field of the Invention
The present invention relates to an information recording medium.
2. Description of the Related Art
The first subject of the present invention is as follows.
Some conventional information recording media are manufactured by forming a film made of, e.g., a GeSbTe alloy on a substrate made of a transparent material, such as an acrylic resin or a polycarbonate resin, and forming a film made of a metal, such as Bi or a BiTe alloy, having a large light absorption coefficient on the GeSbTe alloy film. In performing recording in an information recording medium of this type, laser light is radiated to heat the two films to diffuse the atoms of the materials forming these films, thereby alloying the films. Reproduction is performed by making use of the difference in reflectance between the alloyed recorded region and an unrecorded region.
Generally, a GeSbTe film left intact in a state immediately after formation has an amorphous phase. When this amorphous phase is checked by an X-ray diffraction method (XRD) which is normally used in checking lattice structures, only a broad diffraction peak indicating an amorphous phase is given, but no information about amorphous phases inherent in individual materials can be obtained. Therefore, no information about the state of the amorphous phase of the GeSbTe film can be obtained either. For this reason, the relationship between the GeSbTe film and the recording sensitivity is totally unknown. To check which composition maximizes the recording sensitivity when the GeSbTe film is used as a recording film, it is necessary to form GeSbTe films having different compositions and check the respective resulting recording sensitivities.
Generally, an amorphous phase transforms into a crystalline phase when heated. Therefore, it is possible to estimate the recording sensitivity of an amorphous phase by using the crystallization temperature or the crystallization energy as a parameter without using any detailed knowledge about the amorphous phase. However, since the recording sensitivity estimated by using the crystallization temperature or the crystallization energy as a parameter does not necessarily agree with an actual recording sensitivity, it is not possible to determine a condition in which a high recording sensitivity is obtained. Therefore, it is currently impossible to specify an information recording medium with the highest recording sensitivity.
The second subject of the present invention is as follows.
As discussed above, optical recording is conventionally done by using the difference in reflectance produced between a recorded region and an unrecorded region of a recording film by the change from an amorphous phase to a crystalline phase. As the composition of a recording film, a composition near a stoichiometric composition represented by Ge.sub.2 Sb.sub.2 Te.sub.5, in the case of a GeSbTe alloy, is used even for a recording film used in an overwrite scheme or a recording film which causes diffusion and alloying. Commonly, (Ge.sub.2 Sb.sub.2 Te.sub.5).sub.90 Ge.sub.10 containing Ge in excess by about 10% is used as the composition of this intermetallic compound. A representation of this type is used in order to readily understand how the physical meaning changes from an intermetallic compound composed of Ge.sub.2 Sb.sub.2 Te.sub.5. Note that an intermetallic compound represented by Ge.sub.2 Sb.sub.2 Te.sub.5 is regarded as a mixed crystal of binary compounds, GeTe and Sb.sub.2 Te.sub.3, i.e., as (GeTe).sub.2 (Sb.sub.2 Te.sub.3).
An amorphous structure is generally characterized in that no X-ray diffraction peak is produced from any specific crystal face even when the structure is checked by X-ray diffraction, since there is no translational symmetry in a structure in a corresponding crystalline state. Therefore, X-ray diffraction has been conventionally used to check whether a substance of interest has a crystalline phase or an amorphous phase. However, if a substance to be checked contains chalcogenide elements, such as S, Se, or Te, for example, if a substance to be checked is a GeSbTe film with an amorphous phase, it is only possible to merely infer that elements Ge, Sb, and Te exist in a randomly distributed state. That is, it was only possible to confirm the existence of one structure termed amorphous GeSbTe by performing X-ray photoelectron spectroscopy (Oshima et al., Extended Abstracts (The 37th Spring Meeting, 1990); The Japan Society of Applied Physics and Related Societies, Presentation No. 31p-ZE-1, p.406). Therefore, this result does-not refer much to the presence of a chemical order, although it depends on the compositions, but conclude that elements Ge, Sb, and Te exist at random.
As described above, an information recording medium uses the phase change from an amorphous phase to a crystalline phase or vice versa in recording information. Therefore, since the correspondence of an amorphous structure with a crystal structure is totally unknown, it is impossible to clearly determine in which region high-sensitivity recording can be performed in a composition containing of Ge, Sb, and Te, regardless of whether the recording is of a write once type or an overwrite type; that is, the determination is done by trial and error. This is so because the structure of an alloy containing of Ge, Sb, and Te in an amorphous state is completely unknown.
Using an amorphous-phase GeSbTe alloy film as a recording film has already been reported by, e.g., Jpn. Pat. Appln. KOKAI Publication No. 62-53886 and U.S. Pat. No. 4,670,345. In these patent specifications, the composition of a GeSbTe alloy as the material of a recording film is expressed as (Sb.sub.x Te.sub.1-x).sub.y Ge.sub.1-y wherein x is 0.05 to 0.7 and y is 0.4 to 0.8. The purpose of these disclosures is to improve the thermal stability of Sb.sub.2 Te.sub.3 by adding Ge to Sb.sub.2 Te.sub.3 as a base, thereby raising the crystallization temperature.
These specifications do not at all mention an amorphous state, particularly a chemical bonded state or a cluster molecular state between elements Ge, Sb, and Te. That is, Jpn. Pat. Appln. KOKAI Publication No. 62-53886 or U.S. Pat. No. 4,670,345 does not at all touch upon the relationship between the chemical bonding state and the optical recording sensitivity in an amorphous state. In addition, these patent specifications have disclosed that an information recording medium can also be formed by stacking a GeSbTe film and a second recording film. However, the specifications do not at all refer to the influence that the internal chemical bonding state of the GeSbTe film has on the phase change of the layered film.
Furthermore, these patent specifications do not at all touch upon the effect that Ge--Te and Sb--Te cluster molecules consisting of two elements in the amorphous-phase GeSbTe film have on the recording sensitivity, or the effect that the existing bonds or cluster molecules have on the recording sensitivity. Basically, the expression (Sb.sub.x Te.sub.1-x).sub.y Ge.sub.1-y gives the impression that a chemical bond is present between Sb and Te. Although this is true in at least a crystalline state, it has not been confirmed yet whether this is also true for an amorphous state.